1. Field of the Invention
This invention relates to a lens for image pickup, and in particular to a lens for image pickup suitable for application in image input devices for portable telephones and personal computers, digital cameras, CCD cameras for monitoring, inspection equipment, and other equipment employing a CCD or CMOS as an imaging device.
2. Description of Related Art
A lens for image pickup as described above must have a short optical length, defined as distance from the incidence plane on the object side of the lens for image pickup to the image pickup surface (the image-formation surface of the CCD or similar). Taking a portable telephone as an example, the optical length must be shorter than, at least, the thickness of the portable telephone set. On the other hand, it is preferable that the back focus, defined as the distance from the emission plane on the image side of the lens for image pickup to the image pickup surface, be as long as possible. This is because of the need to insert filters and other components between the imagine lens and the image pickup surface.
Apart from the above, a lens for image pickup is required to be corrected such that various aberrations are reduced by a sufficient amount that distortion of the image is not visually perceptible, and as required by the integration density of the imaging elements (also called “pixels”). Below, “various aberrations have been corrected so as to be sufficiently small that image distortions are not visually perceived, and requirements of the imaging element integration density are met” is, for simplicity, represented by the phrase “various aberrations are satisfactorily corrected” or similar. An image for which various aberrations are satisfactorily corrected may be called a “satisfactory image”.
As a lens for image pickup satisfying the above requirements, lens systems with a small number of component lenses, with short optical length and designed for compactness, have been proposed. However, such lens systems employ lenses which are expensive due to the use of aspherical molded glass, or in which the curvature radius cannot, due to machining constraints, be made small in order to shorten the optical length. Lens systems are also seen in which a single lens is used, in order to achieve a short optical length, so that consequently aberrations cannot be completely eliminated.
One lens for image pickup which resolves the above-described problems has a back focus of appropriate length, a broad angle of field, and consists of two groups of two lens, with small distortion aberration (for example, see Japanese Patent Laid-open No. 2001-174701). There is also a lens for image pickup with a sufficiently long back focus, the refractive powers of the objective-side lens and image-side lens of which can be set appropriately, and with a two-group, two-lens configuration which is easily manufactured (for example, see Japanese Patent Laid-open No. 2000-321489). In addition, there is a lens for image pickup which is small, lightweight, with good telecentric properties, easily corrected astigmatic aberration, and machining and assembly of which is easy (see for example Japanese Patent Laid-open No. 2002-267928).
However, the optical length of a lens for image pickup to be mounted in a portable telephone set designed for compactness must be, at most, approximately 6 mm, and it is also required that satisfactory images be acquired. That is, as portable telephones become increasingly thin, it will become impossible to use a lens for image pickup if it does not have an optical length shorter than that of the lenses for image pickup disclosed in the above three patents, or is not capable of acquiring satisfactory images.
An object of this invention is to provide a lens for image pickup which, while having an F number of approximately 2.8, is configured from a small number of lenses, namely two, has a short lens optical length of 6 mm, and can acquire satisfactory images.
A further object is to provide a lens for image pickup which, by realizing all lenses (two lenses) configuring the lens for image pickup of this invention using plastic material, achieves reduced costs and lighter weight.
Here “a plastic material” is a polymer substance which can be caused to undergo plastic deformation, under heat, pressure, or both, and molded into a lens shape, and which is transparent to visible light.